A broadband X-ray imaging spectroscopy with high-angular resolution:
the FORCE mission
Koji Mori,1 Takeshi Go Tsuru,2 Kazuhiro Nakazawa,3 Yoshihiro Ueda4 , Takashi Okajima5 ,Hiroshi Murakami6 , Hisamitsu Awaki7 , Hironori Matsumoto8 , Yasushi Fukazawa9 , Hiroshi Tsunemi10 ,and Tadayuki Takahashi11
1

Department of Applied Physics and Electronic Engineering, University of Miyazaki, Miyazaki 889-2192, Japan2
Department of Physics, Kyoto University, Kyoto 606-8502, Japan3
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
4
Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
5NASA/Goddard Space Flight Center, MD 20771, USA
6
Department of Information Science, Faculty of Liberal Arts, Tohoku Gakuin University, Miyagi 981-3193, Japan
7
Department of Physics, Ehime University, Ehime 790-8577, Japan
8
Kobayashi-Maskawa Institute, Nagoya University, Aichi 464-8602, Japan
9
Department of Physical Science, Hiroshima University, Hiroshima 739-8526, Japan10
Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
11
ISAS/JAXA, Kanagawa 252-5210, Japan
E-mail(KM): mori@astro.miyazaki-u.ac.jp
Abstract
We present our concept of future Japan-lead medium-class mission, FORCE (Focusing On Relativistic
universe and Cosmic Evolution), to be launched in the mid 2020s. FORCE is the direct successor tothe broadband X-ray imaging spectroscopy aspect of Hitomi (ASTRO-H) with significantly higher angularresolution. The current design of FORCE defines energy band pass of 1–80 keV with angular resolution of <15′′ in half-power diameter, achieving a 10 times higher sensitivity above 10 keV compared to any previous
missions with simultaneous soft X-ray coverage. Our primary scientific objective is to trace the cosmic
formation history by searching for “missing black holes” in various mass-scales: “buried supermassive
black holes (SMBHs)” residing in the center of galaxies in a cosmological distance, “intermediate-mass
black holes” acting as the possible seeds from which SMBHs grow, and “orphan stellar-mass black holes”
without companion in our Galaxy. In addition to these missing BHs, hunting for the nature of relativistic
particles at various astrophysical shocks is also in our scope, utilizing the broadband X-ray coverage withhigh angular-resolution. FORCE are going to open a new era in these fields. The satellite is proposed tobe launched with the Epsilon vehicle that is a Japanese current solid-fuel rocket. FORCE carries threeidentical pairs of Super-mirror and wide-band X-ray detector. The focal length is currently planned tobe 10 m. The silicon mirror with multi-layer coating is our primary choice to achieve lightweight, goodangular optics. The detector is a descendant of hard X-ray imager onboard Hitomi (ASTRO-H) replacing
its silicon strip detector with SOI-CMOS silicon pixel detector, allowing an extension of the low energy
threshold down to 1 keV or even less.
Key words: instrumentation: high angular resolution — X-rays: diﬀuse background — accelerationof particles

1. Scientific objectives and requirement
Our primary scientific objective is to trace the cosmic
formation history by searching for “missing black holes”
in various mass-scales. The missing black holes we define
includes “buried supermassive black holes (SMBHs)”(> 104 M⊙ ) residing in the center of galaxies in a cosmo-logical distance, “intermediate-mass black holes” (102 –104 M⊙ ) acting as the possible seeds from which SMBHs
grow, and “orphan stellar-mass black holes” (< 102 M⊙ )without companion in our Galaxy. In addition to thesemissing BHs, hunting for the nature of relativistic particles at various astrophysical shocks is also in our scope.

In order to achieve these scientific objectives described
here, a high sensitivity in the hard X-ray band with simultaneous broadband coverage is required. The sensitivity to point sources is generally limited by exposure
time, background, and source confusion, and is gettingbetter as exposure time increases, but at some point itsaturates due to source confusion limit. Thus, higher
angular resolution is strongly demanded to achieve ahigh sensitivity. Especially, in order to go down to 2–3
×10−15 erg cm−2 s−1 , a requirement from the AGN survey (Mori et al. 2016), the angular resolution of < 15′′in HPD is necessary.
2.

Mission design and scientific instruments

Fig. 1. A schematic view of the FORCE satellite.

FORCE has a weight of about 1 metric ton, and isplanned to be launched with the Epsilon vehicle thatis an ISAS/JAXA solid-fuel rocket. The satellite will be
put into a circular orbit with altitude of 500–600 km andinclination angle of 31 degrees or less, which is similar to
those of previous Japanese X-ray observatories. Figure 1
shows a schematic view of the satellite. FORCE carries
co-aligned, three identical pairs of a supermirror withhigh angular resolution and a focal-plane detector withbroadband response. The supermirror and the detector
are separated by 10 m focal length. Such a long focal
length is necessary to ensure suﬃcient eﬀective area for
hard X-ray focusing and requires an extendable opticalbench (EOB) that can be stowed to fit in the launchfairing and deployed on-orbit. In the current design, the10 m length is achieved by a combination of 2 m fixed optical bench (FOB) and 8 m EOB, and the mirror moduleis placed at the end of the EOB.
Table 1 summarizes key instrument parameters at thismoment. High angular resolution of < 15′′ in the broadband of 1–80 keV characterizes this mission. These parameters are defined by the design concepts of the mirror
and detector, which are described in the following sections.

Table 1. Instrument parameters

Angular resolution
Multi-layer Coating
Field of view at 30 keV
Eﬀective Area at 30 keVEnergy range
Energy resolution at 6 keV
Background
Timing resolution
Working temperature<15′′ (HPD)
Pt/C∼7′ ×7′ (50% response)
370 cm2
1–80 keV<300 eV (FWHM)
comparable to Hitomi/HXI
several × 10 µs−20±1 ◦ C

2.1. X-ray supermirror with high-angular resolution andWideband hybrid X-ray detector
The mirror substrates of our X-ray supermirrors are
made based on the single-crystal silicon mirror technology, which has a high potential of making light-weight,
high-angular resolution X-ray optics (Zhang et al. 2016).
In general, higher angular resolution is accompanied withlarger mass and higher production cost. Considering ourlimited resource, we need to find out the best compromise among these factors. The silicon mirror can provide
us with a solution for this issue. Although there are still
a number of technical issues to be verified experimentally toward a flight-ready telescope, the silicon mirror
with multi-layer coating is our primary choice to achieve
light-weight, aﬀordable, high angular optics.
The focal-plane detector of FORCE, wideband hybrid
X-ray imager (WHXI), has the same concept as the hard
X-ray imager (HXI) onboard Hitomi; Si and CdTe hybriddetector with active shield. Although HXI consists offour layers of double-sided Si strip detectors (DSSD) anda single layer of CdTe double-sided strip (DSD) detector(Nakazawa et al. 2016), WHXI replaces the four DSSDlayers with a single SOI-CMOS pixel detector (SOIPIX)(Tsuru et al. 2014). Low readout noise achievable bySOIPIX could lower the energy threshold down to 1 keV
and assure a broadband energy response required to thefocal-plane detector of FORCE. CCD is an established Sidetector with low readout noise, but its working temperature, typically < −60 ◦ C, is too low to be placed closely
together with the CdTe DSD, whose working temperature is about −20 ◦ C, in a single camera. SOIPIX has agood time resolution, and what is more, self-trigger function so that anti-coincidence technique can be utilized,
which is necessary to achieve low background.
References
Mori, K. et al. 2016, SPIE, id. 99051O
Zhang, W. et al. 2016, SPIE, id. 99051S
Nakazawa, K. et al. 2016, SPIE, id. 990511
Tsuru, T.G. et al. 2014, SPIE, id. 914412